The invention relates to interleaving techniques. These techniques are generally implemented to reduce the correlation that is introduced by a “selective” filtering operation that is inherent to the transmission channel.
The invention applies in particular to any type of multiple carrier transmission system in which the information is put into the form of data symbols (quadrature amplitude modulated (QAM), quadrature phase shift keyed (QPSK), . . . cells) and is multiplexed on an array of subcarriers corresponding to frequency subdivision of the instantaneous bandwidth of the transmission system, which system is subjected to a frequency disturbance that gives rise to the effect whereby a transmission channel has a transfer function H(f,t) that is highly colored and that varies little over time, or else to a system that is subjected to strong Doppler dispersion that generates a loss of orthogonality between subcarriers of a module for multiplexing and modulating by orthogonal functions. Such a module is referred to as an orthogonal frequency division multiplexing (OFDM multiplex) module for a device that multiplexes modulated symbols on subcarriers that correspond to the conjugate Fourier components of a Fourier transform of size NFFT that corresponds to the number of subcarriers of the OFDM multiplex. On transmission, the useful OFDM signal in its time representation is made up of NFFT samples and, for each sample, is the result of summing complex symbols modulated by the conjugate Fourier components. On reception, projecting the OFDM signal onto the corresponding Fourier components and integrating over all of the samples of the OFDM signal provides an estimate of one of the symbols of the OFDM multiplex. By analogy, a module for multiplexing and modulating the transmitted symbols by orthogonal functions is also referred to below in this document by the term orthogonal multiplex. When the number of data symbols given by the orthogonal multiplex is less than the number of orthogonal functions, a null symbol is placed in the center, and null symbols are placed at the ends of the orthogonal multiplex in symmetrical manner. The transmission channel, also referred to as a multipath channel, is represented by the impulse response h(t,□) of a digital filter in which t represents the time variable and □ represents the delay variable associated with the coefficients of the filter at instant t. The transmission channel filters the multicarrier signal by weighting each symbol by the transfer function of the channel as resolved onto the OFDM multiplex. On reception, it generates correlations of the subcarriers in the frequency and time domains. The frequency correlation affects the subcarriers, and the time correlation gives rise to subcarriers of amplitude that is quasi-constant over an observation window having a duration of the same order as the coherence time of the channel. The coherence time corresponds to the mean value of the time difference needed to ensure decorrelation between the signal representative of the transmission medium and its time-shifted version.
These two correlations put a limit on the performance of decision circuits on reception.
The time correlation gives rise to bursts of errors after deciding transmitted data symbols and after decoding estimated transmitted bits. These effects are encountered when the environment is varying slowly and is a multipath environment. This applies in particular for ultra wide band (UWB) systems, for radio systems dedicated to the radio local loop (Wi-max and digital enhanced cordless telecommunications (DECT)), or for transmission of the xDSL type (DSL: digital subscriber loop).
Frequency correlation is the result of the multipath effect that introduces filtering, of the Doppler effect, and of phase noise in the radiofrequency (RF) stages acting simultaneously and giving rise to a loss of orthogonality between the subcarriers of an orthogonal multiplex. This applies in particular to short-range systems defined in the millimeter band as specified by the American Standards Organization in IEEE802.15.3c, and also in highly mobile systems or in very long-range systems dedicated to ionospheric radio links (Digitale Radio Mondiale (DRM) system standard ETSI TS 101 980).
A method of remedying those two correlations consists in performing interleaving on transmission that is performed on the binary data or on the data symbols.
Interleaving techniques in a transmission system are thus applied to the data items in order to decorrelate the data items as received and improve the decision-making circuits.
At binary level, when the system is associated with a redundancy device, the interleaving techniques applied after the redundancy operation serve to reduce the size of the error bursts. Interleaving is said to be “binary” when it applies to encoded bits, or to bits extracted directly from the source, which bits are referred to as scrambling bits.
Interleaving is said to be “frequency” interleaving when it applies to complex symbols (QPSK, x-QAM, . . . ) allocated to the subcarriers of an orthogonal multiplex, and its size is equal to the number of data symbols per orthogonal multiplex. Interleaving is always performed on the useful data of the transmission device with a static interleaving law for each transmission mode defined by the number of modulation states, the type of encoding, etc. . . . The term “useful” data is used to mean the transmitted data conveying an information message and not including the data that is dedicated to signaling and identification. Below, in this document, the term “data” is used to designate useful data.
The invention relates more particularly to frequency interleaving, i.e. to interleaving performed in the frequency domain on symbols allocated to the carriers of an orthogonal multiplex. This type of interleaving takes place at the input to an orthogonal multiplex. In equivalent manner, reference is commonly made to carrier or subcarrier interleaving.
Document ETSI 300 401, “Radio broadcasting systems: digital audio broadcasting (DAB) to mobile, portable, and fixed receivers”, May 1997, p. 182, gives a description of the frequency interleaver of the DAB multicarrier system. That constitutes frequency type interleaving performed at the scale of an OFDM multiplex since the permutation law P(i) takes account of the size NFFT of the FFT of the OFDM multiplex. It is applied to the data symbols of the system corresponding to QPSK symbols.
For a given transmission mode, the interleaving law transforms a QPSK symbol q1,n into a new symbol y1,k where k is the index of the carrier after interleaving, n is the index of the carrier before interleaving, and 1 is the number of the OFDM symbol in the DAB frame transmitted at instant (1-1)TSYM, where TSYM designates the total duration of an OFDM symbol together with a guard interval. The interleaving law applied to the symbols allocated to the carriers of the OFDM multiplex and specified by the subcarrier interleaving law, is a law of the form k=F(n) where k is the index of the data carrier in the OFDM multiplex. The index k varies over the range {−Npm/2, Npm/2}\{0} and the index n varies over the range [0, Npm−1]. Npm corresponds to the number of data subcarriers per OFDM multiplex. The interleaving law is extracted from an alphabet A that takes account of the size NFFT of the OFDM multiplex. An interleaving law P(i) is defined initially for i={0, NFFT−1} taking the values A={P(0), P(1), . . . , P(NFFT−1)} in the integer space I={0, NFFT−1} in application of a law P(i) having the form:
                                          P            ⁡                          (              0              )                                =          0                ⁢                                  ⁢                                            P              ⁢                              (                i                )                                      =                                          [                                                      13                    ·                                          P                      ⁡                                              (                                                  i                          -                          1                                                )                                                                              +                                      (                                                                                            N                          FFT                                                4                                            -                      1                                        )                                                  ]                                            N                FFT                                              ,                                          ⁢                      i            =                          {                              0                ,                                                      …                    ⁢                                                                                  ⁢                                          N                      FFT                                                        -                  1                                            }                                                          (        1        )            
The operation [X]NFFT corresponds to the modulo NFFT operation that provides the remainder of dividing X by NFFT. The values P(i) that are strictly less than Q=(NFFT−Npm)/2 and that are strictly greater Npm+Q are eliminated, as is the value NFFT/2 that corresponds to the central carrier.
The data is then distributed in increasing order i in a vector D={d0, d1, . . . , DNpm-1} taking its values in the interval I′={0, . . . , Npm−1}.
The correspondence F(n)=dn−NFFT/2 is performed to distribute the subcarriers between the indices {−Npm/2 and Npm/2} excluding the index 0 for the central carrier. The process of interleaving the subcarriers corresponds to the diagram of FIG. 1. This figure shows the general principle of interleaving subcarriers in conventional systems of the OFDM type.
The interleaved data symbols are then put into frames and then distributed over the OFDM multiplex prior to transmission. The transmission channel filters the transmitted signal, thereby giving rise to correlation between data symbols. On reception, the deinterleaving operation on the data symbols upstream from the decision circuits then makes it possible to obtain data with reduced correlation at the input to the decision circuits. Nevertheless, for corresponding systems, the interleaving period is generally short compared with the coherence time of the channel and error bursts persist at the output from the decision circuit.